Apparatus for detecting ferromagnetic objects and screeing people and equipment

ABSTRACT

An apparatus for detecting a ferromagnetic object located on or in a person being screened includes a first magnetic sensor which measures an ambient magnetic field or gradient within a first volume of space and produces a corresponding measurement signal, a primary power supply provides power to the magnetic sensor, a signal processing circuit is in communication with the magnetic sensors identifies temporal variations in the measurement signal and from the identified temporal variations provides an output signal indicative of the presence of a ferromagnetic object within the volume of space, and a warning device operable by the output from the signal processing circuit provides in the vicinity of the apparatus an audible and a visible warning in response to the output signal from the signal processing circuit. The apparatus may include a user operable input means that disables the warning device without powering down the magnetic sensors.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/129,877, filed Mar. 31, 2014, which is the National Stage ofInternational Patent Application No. PCT/GB2012/051500, filed Jun. 27,2012, which claims priority to GB1111067.3. All applications areincorporated herein expressly by reference.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

The present invention relates to apparatus for detecting the presence offerromagnetic objects on a person, in particular to apparatus for use inpre-screening of patients before they enter a room containing a magneticresonance imagining (MRI) scanner. It also relates to a method ofscreening a patient.

In GB 2 395 276 there is taught an apparatus which is able to detectferromagnetic objects by providing a primary sensor means comprisingfirst and second passive magnetic sensors which detect the disturbancesin the ambient magnetic field which occur as the object moves throughthe field. The apparatus also includes a secondary non-magnetic sensormeans which detects movement of objects in the vicinity of the primarysensor means. If both the primary and secondary sensing means detect amoving object an alarm is triggered. It is proposed in that patent thatthe apparatus may be mounted to the wall on either side of a doorway toa room containing a magnetic resonance imaging apparatus, with thesecondary sensor means being arranged to detect objects approaching orabout to pass through the doorway. Because both the primary andsecondary sensors must detect the object this arrangement helps toreduce false alarms.

The apparatus described above works very well to warn people who may beunintentionally about to take a ferromagnetic object into an MRI room.It is known that there have in the past been several unfortunateaccidents which have occurred due to a ferrous object entering an MRIroom and being magnetically propelled, at high velocity, into the MRImachine within the room. This is called the projectile effect, and iswell described in GB 2 395 276.

Ideally, before a person attempts to enter a room containing an MRImachine they will have been screened for magnetic objects. This requiresan MRI qualified person to carry out a screening procedure on thepatient before they approach the room. Combined with the use of anoptional detector at the door to the room, the risk of the person takinga magnetic object into the room is greatly reduced.

Furthermore, a patient may inadvertently be carrying ferromagneticobjects that are not substantial enough to be a projectile effecthazard. Such objects may cause degradation in the MRI image and requirethe patient to be re-scanned. This reduces the efficiency of the MRIscanning process in hospitals. Examples of objects in this categoryinclude hair/bobby pins, wrist-watches and jewelry clasps and pins.

According to a first aspect the invention provides apparatus fordetecting a ferromagnetic object located on or in a person beingscreened, the apparatus comprising:

a first magnetic sensor which in use measures an ambient magnetic fieldor gradient within a first volume of space and produces a correspondingmeasurement signal,

a primary power supply which provides power to the magnetic sensors,

a signal processing circuit arranged in communication with the magneticsensors configured to identify temporal variations in the measurementsignal and from the identified temporal variations provide an outputsignal indicative of the presence of a ferromagnetic object within thefirst volume of space,

and

a warning device operable by the output from the signal processingcircuit to provide within the vicinity the apparatus at least one of anaudible and a visible warning in response to the output signal from thesignal processing circuit,

and characterized in that the apparatus include a user operable inputmeans which enables the warning device to be disabled by a user withoutpowering down the magnetic sensors.

Most preferably the apparatus also includes a second magnetic sensorwhich in use measures an ambient magnetic field or gradient within asecond volume of space that at least partially overlaps the first volumeof space and produces a corresponding measurement signal, and signalprocessing circuit is arranged in communication with the first andsecond magnetic sensors and is configured to identify temporalvariations in the measurement signal and from the identified temporalvariations provide an output signal indicative of the presence of aferromagnetic object within the overlapping regions of the two volumesof space. This can be readily extended to three or four or more magneticsensors.

By an overlapping region of two localized volumes of space we mean aspace in a known region which is close to the apparatus, typicallywithin 1 meter of the apparatus within which a person to be screened canbe located. The space should be large enough to fully enclose a personof 99th percentile height or greater when standing by the apparatus,Each of the sensors may respond to ferromagnetic objects that are movingoutside of the that space, but generally will be less sensitive forobjects that are further away.

The signal processing circuit may include one or more filters and theoutput signals from the two sensors may be passed through the one ormore filters, The filters may include a low pass filter, and may includea high pass filter. These may be combined in a band pass filter. Thehigh pass filter may be configured to remove frequencies above, say, 10Hz which typically correspond to changes in magnetic field caused bynearby electrical appliances. The low pass filter may be configured toremove frequencies below, say, 0.2 Hz, which mainly correspond to thebackground magnetic field produced by the earth which changes veryslowly over time.

Where the output of the sensors is passed through a filter the useroperable input means may enable the warning device to be disabledwithout powering down the filter. Such filters can take a long time tostabilize, so ensuring they are not powered down can reduce the timetaken for the apparatus to be enabled when the user operable input meansis operated. Disabling the warning device removes the unwanted feelingexperienced by some MRI qualified personnel that they are constantlybeing monitored, and prevents false alarms being issued when they passclose to the apparatus with ferromagnetic objects during their day today work when they are not in fact using the apparatus to screen apatient.

Operating the user operable input means may alter the value of arelatively low current electrical signal, the apparatus disabling orenabling the warning device according to the value of that signal. Byrelatively low current electrical signal we mean a signal of less than 1mAmp such as a digital logic signal whereby the input means changes thelogic level of the signal according to whether the user has enabled ofdisabled the warning device. By relatively low current we mean a currentthat is considerably lower than the current drawn by the signalprocessing means or warning device.

The signal may be carried from the user operable input device alongtwisted pair cables.

The applicant has appreciated that it is desirable to minimize thecurrent flowing through cables to and from the input device because thechanging signal may generate a magnetic flux which might be picked up bythe sensors leading to false warning signals being generated. The lowerthe current the smaller the field and so the less likely this is tohappen.

The apparatus may include a second power supply which provides at anoutput power for the warning device, the output of the power supplybeing switchable between a level at which the power to the warningdevice is enabled and a level at which power to the warning device isdisabled, the condition of the output being dependent on the value ofthe signal from the user operable input means. For instance, the powersupply may include a digital control circuit which may receive thesignal from the user operable input means at an input terminal whichcauses the output of the power supply to switch on or off.

The second power supply may be switchable independently of the firstpower supply, ensuring that the power to the magnetic sensors and thefilters (where present) is not interrupted.

The secondary power supply may be fed from a power output of the firstpower supply. Both the first power supply and the second power supplymay comprise DC-DC converters and each may provide a dual polarity DCoutput signal, having a positive and negative output voltage,

A third power supply may be provided. This may provide power to thefirst power supply and the second power supply where present. It maycomprise a battery, or may comprise an AC-DC converter which may take aninput from a mains supply and provide as an output a DC signal suitablefor input to the other power supplies.

The output of the power supply (supplies) may be connected to the signalprocessing means and warning device through a twisted pair cable. Thismay comprise two conductive cables which are surrounded by respectivelyinsulating sleeves and which are then twisted together along theirlength. Each conductive cable may in turn comprise two individualstrands of conductive wire which are twisted together along their lengthwithin the insulating sleeves. The twisting helps to minimize themagnetic flux that is radiated from the cable towards the sensors,reducing or eliminating false warnings.

In an alternative to the user operable input means producing arelatively low level signal that is used to turn the output of asecondary power supply on or off, the user operable input means couldcomprise a switch that is arranged in series between at least one outputof the first power supply, or second power supply where present, and thewarning device. Therefore, rather than a low level signal the userdevice will interrupt the relatively higher current supply fed to thewarning device. This, however, for the reasons already stated may not bepreferred depending on the relative positions of the power supply andthe user operable input means as it is desirable to minimize the amountof cable within the apparatus along which high currents travel, andclearly the power would need to run from the location of the powersupply to the switch and then on to the warning device. In any event,even if the user operable input means and power supply are convenientlyclose together it is generally not desirable to tap into the outputcables from the power supply as this can create unwanted localizedmagnetic fields that may cause false warnings.

The apparatus may comprise a single housing which may be pole shaped,and would normally be mounted on a wall when in use or optionally in afree standing base which supports the housing above the ground with themagnetic sensors being arranged within the housing. It may comprise asingle housing, which may be elongate and may be shaped like a pole,which includes both the first and second magnetic sensors. The housingmay define a single void within which the signal processing means,sensors and power supply (supplies) are provided but could define two ormore voids which may be interconnected. The power supply (supplies),sensors, signal processing means and optionally the warning means may belocated within the pole.

When in use (mounted on a wall or floor standing) the user operableinput means may be located on the housing at a height where it can bepressed by a user of average height (between 5 and 6 foot tall) withouthaving to stoop down, and the warning device may be located at the topof the housing.

The magnetic sensors may have an overlapping detection zone which liesto the front of the housing and encompasses a volume which extends fromfloor level up to at least 2.1 meters and outward from the housing by atleast 1.2 meters. This defines a zone which will encompass the whole ofa 99th percentile person standing in front of the housing, so that anyferromagnetic objects they are carrying, wearing or have inside theirbody will be detected by the signal processing means,

The apparatus may include a mat which can be positioned on the ground infront of the housing and which indicates where a person must stand to bewithin the localized zone. The mat may carry indicia showing where theperson should place their feet on the mat.

The user operable input means may be located at the front side of thehousing. This allows a person standing in the sensitive zone toenable/disable the warning device in a convenient manner.

Where the housing comprises a pole, one of the magnetic sensors may belocated at, or close to the top of the pole and the other one at orclose to the bottom of the pole, and the input may be locatedconveniently at or close to the midpoint of the pole. They may begenerally aligned with a vertical axis that passes through both sensors.

The user operable input means may comprise a button or a switch. It maycomprise a non-contact type switch such as a proximity sensor, perhapsan optical sensor, which allows the switch to be operated withoutcontact by a user.

The first and second sensors and the signal processing means may beconfigured as a gradiometer.

In one arrangement, the apparatus may include a third sensor and thefirst, second and third sensors may be configured as a second ordergradiometer. The third sensor may be located towards the middle of thepole, midway between the first and second magnetic sensors. All threesensors may be aligned with a common vertical axis. The use of threesensors may provide an apparatus which is relatively more sensitive tonearby ferromagnetic objects and less sensitive to distant objects andwhich gives a larger change in output for a given movement of a nearbyobject. This is especially advantageous in a busy or compact environmentwhere the potential for interfering magnetic fields from people andequipment moving nearby is high. In addition, it is also advantageousfor an apparatus that may be temporarily located within a room, as lesstime has to be spent ensuring that it is positioned far enough away frompotential interfering magnetic fields that could produce false warnings.With a less sensitive two sensor apparatus more care is needed duringset up.

According to a second aspect the invention provides a method ofscreening a person or object comprising:

providing an apparatus comprising a first magnetic sensor which in usemeasures an ambient magnetic field or gradient within a first volume ofspace and produces a corresponding measurement signal,

a primary power supply which provides power to the magnetic sensors,

a signal processing circuit arranged in communication with the magneticsensor configured to identify temporal variations in the measurementsignal and from the identified temporal variations provide an outputsignal indicative of the presence of a ferromagnetic object within thevolume of space,

and

a warning device operable by the output from the signal processingcircuit to provide within the vicinity the apparatus at least one of anaudible and a visible warning in response to the output signal from thesignal processing circuit,

the method comprising the steps of:

(a) Positioning the person or object to be screened within the volume ofspace in a first orientation,

(b) Positioning the person or object in the volume of space in a secondorientation which is at least 180 degrees out of alignment with thefirst position, and

(c) Observing any warning from the warning device.

The method ensures that ferromagnetic items that are on a person orobject being screened are always brought close to the sensors and assuch only a single apparatus providing single-sided screening isrequired. This negates the need for having two apparatus forming aportal separated by enough distance for a person to pass through inorder to scan both sides of the person.

The method may comprise a step of rotating the person or object throughat least 180 degrees from the first orientation to a second orientation.

The step of rotating the person may comprise rotating them when they arepositioned in the volume of space. It may rotate the patient through atleast 360 degrees within that space. The apparatus may, at all timesduring the rotation, be switched on so that it is able to detect thepresence of ferromagnetic objects about or in the patient.

The apparatus may include a user operable input which permits thewarning device to be disabled without powering down the magnetic sensorsand the method may comprise an additional step of enabling the displayusing the user operable input means. The apparatus may be in accordancewith the first aspect of the invention.

Upon completion the method may comprise disabling the display withoutpowering down the display using the user operable input means.

The method may include a step of introducing a magnetic object in frontof the device after the display is powered up to test the display priorto position patient.

The method may include providing a mat and positioning the person on themat, and rotating the person whilst the person is standing on the mat,the mat being positioned below the volume of space, and preferablywholly within the volume of space.

The method may comprise carrying the person in a non ferromagneticportering means, such as a wheelchair, through the sensing volume ofspace of the apparatus first in one direction and then in the oppositedirection in both cases close to the apparatus such that both sides ofthe person are scanned.

The method, although described in terms of screening a person, mayequally be applied to the screening of an item of equipment by placingthe equipment close to the apparatus and rotating it through 360degrees.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings. There will now bedescribed, by way of example only, one embodiment of the presentinvention with reference to the accompanying drawings of which:

FIG. 1 is a general view of an embodiment of a screening apparatus inaccordance with the invention;

FIG. 2 is a schematic showing the electric circuit of the apparatus ofFIG. 1;

FIG. 3 is a schematic showing an alternative electric circuit of theapparatus of FIG. 1;

FIG. 4 is an illustration of the arrangement of two sensors in theapparatus of FIG. 1 connected as a first order gradiometer;

FIG. 5 is an illustration of the arrangement of three sensors of thearrangement of FIG. 1 connected as a second order gradiometer; and

FIG. 6 is a flow chart showing the steps performed when carrying out amethod of screening a patient using apparatus of the kind shown in FIG.1.

DETAILED DESCRIPTION

Referring to FIG. 1, a screening apparatus 1 for use in pre-screeningpatients prior to their undergoing an MRI scan comprises an elongatepole shaped housing 2, about 1.5 meters in length which is secured to awall. The housing 2 contains an electronic circuit as will be describedin relation to FIGS. 2 and 3.

The electronic circuit includes a number of sensors which detect thepresence of magnetic objects in close proximity to the housing. The topof the housing 2 includes a warning device 4 in the form of a visible.The display 4 comprises a beacon with a set of three colored lightswithin a transparent housing, allowing the display to emit red, amber orgreen light. It also contains a bar graph display to indicate themagnitude of the detected signal. A user operable input device in theform of a push button 5 is provided midway up the front of the housing 2in a position where it can be conveniently operated by a person withouthaving to stoop down. In use, as will be explained hereinafter, thedisplay 4 provides a visible indication of the presence of a magneticobject 6 in proximity of the apparatus as detected by the sensors.

In more detail, and with reference to the generalized circuit diagramillustrated in FIG. 2 of the accompanying drawings, the electroniccircuit comprises first a main power supply unit PSU 1, which receivesincoming power from a remote power supply (not shown but denoted by thephrase “power in”). This may comprise an AC-DC power pack that islocated outside of the housing and connects through a power lead andplug to a suitable electrical outlet. The PSU1 converts the incomingpower to positive and negative DC voltages of +12v/−12v, which areoutput to positive and negative supply lines in the housing.

Three sensors 10,11,12 are provided in the housing, although in somearrangements there may be only two sensors. The sensors each comprise asensitive flux gate magnetometer which is sensitive to changes in fluxin a localized region of space in the vicinity of the sensor. Thesensors are all sensitive over a common overlapping region of space.They are more sensitive to objects very close to the sensor within thatspace than they are to objects further away in that space. Other typesof sensor could be provided, such as magneto-resistive sensors or HallEffect sensors, or a galvanic coil sensor. Each sensor 10,11,12 outputsa respective measurement signal that is a measurement of the magneticfield incident upon the sensor 10,11,12. The measurement signal fromeach sensor is passed to a signal processing device forming readoutelectronics 13. In an alternative arrangement, two sensors could beprovided but three are preferred as they can be configured to operate asa second order gradiometer. The configuration required for both thepreferred embodiment with three sensors and the alternative with twosensors is illustrated in FIGS. 5 and 4 respectively, where {circumflexover (n)} is a unit vector defining the sensing direction of a sensorand B represents the magnetic field present at the sensor.

With two sensors the sensors are arranged to sense in oppositedirections and their outputs are summed as shown in FIG. 4. Analternative would be for the sensors to sense in the same direction andtheir outputs differenced (not shown) as this is equivalent. In eitherof these ways they are connected so as to define a first ordergradiometer whose output is of the form:

Output∝(B ₁ −B ₂).{circumflex over (n)}

and if {circumflex over (n)} defines the x direction (extending radiallyaway from the pole) and y defines a baseline extending vertically downthrough the sensors the output is therefore the first order derivativeof the field component of B in the x direction with respect to the ydirection, often expressed as ∂B_(x)/∂y

With the preferred three sensors the sensors are arranged with B1 and B3sensing in the same direction and B2 sensing in the opposite directionwith a relative gain of two with respect to B1 and B3. The outputs aresummed as shown in FIG. 5. Alternatively, the sensors could all sense inthe same direction with B2 given a relative gain of minus-two. Witheither way the output is of the form:

Output=(B ₁ +B ₃−2B ₂).{circumflex over (n)}

and if {circumflex over (n)} defines the x direction (extending radiallyaway from the pole) an y defines a baseline extending vertically downthrough the sensors the output is therefore the second order derivativeof the field component of B in the x direction with respect to the ydirection, often expressed as ∂²B_(x)/∂y², giving a much higheruniformity in the y direction and a much lower sensitivity to distantobjects relative to the same object position at a distance compared withthe two sensor arrangement.

Since the apparatus will typically be fixed in position when in use formost of the time the sensors 10,11,12 will register a largely unchangingmagnetic field due to the earth, and unchanging first and second ordergradients. These constitute a large offset on the output of the sensor.This constant offset can be removed using a high pass filter in thereadout electronics 13. The sensors will also likely measure regularchanges in the magnetic field associated with the power supply forelectrical equipment located near the sensors which will cause theoutput to vary at the supply frequency and its harmonics. This can alsobe filtered out using a low pass filter in the signal processing device.The filters 14 collectively constitute a band-pass filter 14 to performthese functions. The filtered output of the filters will therefore takea low value, ideally zero, in its steady state.

The sensors and readout electronics receive power from PSU1. Dependingon the complexity of the filters 14 that are used there may be aconsiderable delay between the sensors 10,11,12 and readout electronicsbeing switched on and the filtered output signal settling to a steadystate. It is therefore important that the power to the sensors 10,11,12and the readout electronics 13, at least the filtering stage 14, is kepton at all times when the apparatus might be imminently be needed forpre-screening.

If a ferromagnetic object carried, or pulled or pushed, by a personclose to the sensors 10,11,12, the ambient magnetic field will bealtered causing a change in the output of the sensor 10,11,12. Thatchange will pass through the filter 14 and be amplified by an amplifierwithin the readout electronics. In order to trigger an alarm the signalsize is compared with a preset threshold. Because the signal may bepositive or negative, the threshold (set by a passing the signal througha threshold detector within the readout electronics) consists of arectification stage followed by a comparator that has a circuit toprovide a threshold voltage. Alternatively, separate comparators areused for positive and negative signals with the outputs combined to givea single alarm signal instead of a rectifier and a single comparator. Anoptional latch (not shown) may be provided which holds the value of thesignal output from the comparator for a predetermined period—perhaps upto 1 second.

The output of the comparator is arranged to have logic level zero forthe state where the signal does not exceed the threshold, and level‘one’ for the state when the signal has exceeded the threshold. Once anobject has passed out of range of the sensors 10,11,12, the logic levelreturns to zero once the signal level has dropped below the threshold.In practice, it may be preferable that the alarm continues for anelapsed time defined by a reset delay and a latch such as a flip-flopthat maintains the output at logic one until the switch/button 5 ispressed.

The output of the latch is passed from the readout electronics to aninput of the warning device, which comprises a display electronicscircuit 15 that drives the display 4. It has been found to bebeneficial, although not essential, that both a visual and audible alarmare provided.

In addition the rectified magnetic signal that feeds into the comparatormay pass to the warning device to control a bar graph indicator.

If the sensors have a digital output or if a digitizer is placedimmediately after the sensors then the electronic functions describedincluding the filters, rectifier, comparator, latches and delays may bedone with a digital processor rather than in analog electronics.

The warning device 15, 4 is powered from a second power supply PSU2. Thepower supply PSU2 has an input port 17 which receives a signal from theuser operable device and depending on the state of the signal will powerup or power down the display electronics. When powered down the display4 will not issue a warning regardless of the value of the latched signaloutput from the readout electronics 13. Notably, PSU2 can be powereddown without powering down PSU1, so that the warning device can bedisabled without cutting power to the sensors 10,11,12 and filters 14.

A modified electronic circuit which can be used is shown in FIG. 3 ofthe drawings. Where parts are the same as those used in the circuit ofFIG. 2 the same reference numbers have been used for clarity, and theassociated description above applies. The key difference is the omissionof the second power supply unit PSU2, and the location of the switch 6in the power supply line from PSU1 to the display electronics. Thus,rather than switching a relatively low current that in turn disables apower supply to the display electronics, the switch breaks the currentflowing to the display electronics directly. This has the disadvantagethat a break in the power supply line could cause unwanted magneticfields to be generated unless care is taken during assembly, but doeshave the benefit of reducing the number and complexity of the componentsthat are needed.

In use, the apparatus may be located at a convenient position in apre-screening area. A mat 7 is positioned in front of the pole 2, whichis in turn oriented so that the user input device faces the mat 7. Theapparatus is then connected to a mains electricity supply (unless itcontains its own battery power source). As soon as the filters havesettled it is ready to use.

A method of using the apparatus to pre-screen a patient is as followswith reference to FIG. 6 of the accompanying drawings:

Initially, before a person is screened a trained MRI person will takethat person through a defined pre-screening procedure. First, thepatient changes 100 into suitable clothing and removes any metal objectsthat they may be wearing such as jewelry, hair grips and so on. They arethen asked to complete 200 a screening questionnaire which asks fordetails such as their medical history and whether they have any metalimplants such as surgical pins. These preliminary stages may vary fromhospital to hospital according to their local procedures so 100 and 200are by way of example. Once this has been completed to the satisfactionof the trained MRI person, the MRI person will ensure that the warningdevice is enabled by pressing 300 the button 5 to switch on thescreening apparatus. In fact, they are simply turning on the warningdevice as the apparatus will previously have been switched on so that itsettles in its steady state ready for use. It has been found that mosttimes MRI persons prefer the warning display 4 to be disabled, as itotherwise may raise many false alarms as they pass close by and can makethem feel that they are being constantly monitored. Furthermore thevisual alarm can be distracting when the apparatus is not in use.

After the warning device 4 is enabled, the patient to be screened isinvited to stand on the mat 7 in front of the pole with their arms bytheir side. They will then be asked to rotate 400 through a full circle(360 degrees) whilst remaining standing on the mat 7.

When the person being screened moves onto the mat 7, the presence offerromagnetic items on or in their body will trigger the warning devicebecause the objects are moving and causing a change in the magneticfield within the overlapping detection zone. Additionally, as the personrotates the objects will trigger 500 the warning device because themagnetic field changes. If the person has a very small ferromagneticobject which is initially hidden from the device because it ispositioned, say, on the opposite side of the person's body from thedevice, it will move closer to the sensors as the person rotates andwill trigger the warning device as it moves.

Once the person has rotated through 360 degrees they may then step offthe mat. If no alarm was raised they can be cleared 700 to approach anMRI room. If an alarm is raised the person is asked 600 to identify anymetal objects and remove them and a more detailed search of their bodymay be carried out to locate the ferromagnetic object that set off thewarning device. The step of rotating the person in front of theapparatus is then repeated until no warning sound is issued. The patientis then deemed safe to proceed 700 to the MRI scanner room and themethod of pre-screening ends 800.

A modified step 400 is required if the patient is non-ambulatory. Inthis case the patient would be screened in a non-magnetic porteringchair, wheelchair or gurney (known as transfer equipment). Then thepatient is pushed over the mat 7 in one direction and then rotatedthrough one half turn (180 degrees) and pushed back past the apparatusover mat 7 so the other side of the patient is also scanned. Thisrequires the MRI person to be free of ferromagnetic material so thatneither the person nor the transfer equipment causes an alarm.

This method may be used to determine whether an object of unknowncomposition contains ferromagnetic material prior to being taken into anMRI room. Examples of items that could be tested are cylinders, patientmonitoring equipment, tools, cleaning equipment, or anything where thereis uncertainty as to whether it would be safe or a potential projectileeffect hazard.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Apparatus for detectinga ferromagnetic object located on or in a person being screened, theapparatus comprising: At least a first magnetic sensor which in usemeasures an ambient magnetic field or gradient within a first volume ofspace and produces a corresponding measurement signal, a primary powersupply which provides power to the magnetic sensor, a signal processingcircuit arranged in communication with the magnetic sensors configuredto identify temporal variations in the measurement signal and from theidentified temporal variations provide an output signal indicative ofthe presence of a ferromagnetic object within the volume of space, and awarning device operable by the output from the signal processing circuitto provide within the vicinity the apparatus at least one of an audibleand a visible warning in response to the output signal from the signalprocessing circuit, and characterised in that the apparatus include auser operable input means which enables the warning device to bedisabled by a user without powering down the magnetic sensors. 2.Apparatus according to claim 1 in which the signal processing circuitincludes one or more filters and in which the output signal from thefirst sensor is passed through at least one of the filters.
 3. Apparatusaccording to claim 2 in which the user operable input means enables thewarning device to be disabled without powering down the filters. 4.Apparatus according to any preceding claim in which the user operableinput means alters the value of a relatively low current electricalsignal, the apparatus disabling or enabling the warning device accordingto the value of that signal.
 5. Apparatus according to claim 4 in whichthe signal is carried from the user operable input device along twistedpair cables.
 6. Apparatus according to claim 4 or claim 5 which includesa second power supply which provides at an output power for the warningdevice, the output of the second power supply being switchable between alevel at which the power to the warning device is enabled and a level atwhich power to the warning device is disabled, the condition of theoutput being dependent on the value of the signal from the user operableinput means.
 7. Apparatus according to any preceding claim in which theoutput of the power supply is connected to the signal processing meansand warning device through a twisted pair cable.
 8. Apparatus accordingto any preceding claim which comprises a single housing within which thesignal processing means, sensors and power supply are located. 9.Apparatus according to claim 8 whereby when in a position of use theuser operable input means is located on the housing at a height where itcan be pressed by a user of average height without having to stoop down.10. Apparatus according to claim 8 or 9 which includes a mat which inuse is positioned on the ground in front of the housing and whichindicates where a person must stand to be within the volume of space.11. Apparatus according to claim 8, 9 or 10 in which the housingcomprises a pole, one of the magnetic sensors is located at, or close tothe top of the pole and the other one at or close to the bottom of thepole, and the use operable input means is located conveniently at orclose to the midpoint of the pole.
 12. Apparatus according to anypreceding claim in which the user operable input means comprises abutton or a switch.
 13. Apparatus according to any preceding claim whichfurther includes a second magnetic sensor which in use measures anambient magnetic field or gradient within a second volume of space thatat least partially overlaps the first volume of space and produces acorresponding measurement signal and in which the signal processingcircuit is configured such that the first and second sensors areconfigured as a gradiometer.
 14. Apparatus according to any precedingclaim which includes a third sensor and the first, second and thirdsensors are configured as a second order gradiometer.
 15. A method ofscreening a person or object comprising: providing an apparatuscomprising a first magnetic sensor which in use measures an ambientmagnetic field or gradient within a first volume of space and produces acorresponding measurement signal, a primary power supply which providespower to the magnetic sensors, a signal processing circuit arranged incommunication with the magnetic sensors configured to identify temporalvariations in the measurement signal and from the identified temporalvariations provide an output signal indicative of the presence of aferromagnetic object within the volume of space, and a warning deviceoperable by the output from the signal processing circuit to providewithin the vicinity the apparatus at least one of an audible and avisible warning in response to the output signal from the signalprocessing circuit, the method comprising the steps of: (a) Positioningthe person or object to be screened within the volume of space in afirst orientation, (b) Positioning the person or object in the volume ofspace in a second orientation which is at least 180 degrees out ofalignment with the first position, and (c) Observing any warning fromthe warning device.
 16. The method of claim 15 which comprises a step ofrotating the person or object through at least 180 degrees from thefirst orientation to the second orientation
 17. The method of claim 16in which the step of rotating the person comprises rotating them whenthey are positioned in the volume of space.
 18. The method of claim 17in which the step of rotating the patient comprises rotating the patientthrough at least 360 degrees within the volume of space.
 19. The methodof claim 17 or claim 18 which includes providing a mat and positioningthe patient on the mat, and rotating the person remains standing on themat, the mat being positioned below the volume o space, and preferablywholly within the zone.
 20. The method of any one of claims 15 to 18 inwhich the apparatus includes a user operable input which permits thewarning device to be disabled without powering down the magnetic sensorsand the method comprises an additional step (d) of powering up thedisplay using the user operable input means.
 21. The method of claim 20which further comprises upon completion disabling the display withoutpowering down the magnetic sensors or signal processing means using theuser operable input means.
 22. The method of any one of claims 15 to 21which includes a step of introducing a magnetic object in front of thedevice after the display is powered up to test the display prior toposition patient.
 23. The method of any one of claims 15 to 22 wherebythe patient is carried in a non ferromagnetic portering means past theapparatus first in one direction and then in the opposite direction inboth cases close to the apparatus such that both sides of the patientare scanned.
 24. Apparatus for detecting a ferromagnetic objectsubstantially as described herein with reference to and as illustratedin the accompanying drawings.
 25. A method of screening a person orobject substantially as described herein with reference to and asillustrated in FIG. 6 of the accompanying drawings.